The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, and three-piece. One-piece balls are molded from a homogeneous mass of material upon which is molded a dimple pattern. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance, especially in terms of the distance such balls travel, and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer, a so called “three-piece ball” results, and similarly, if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
Balata was used as the primary material for covers of golf balls until the 1960's when SURLYN®, an ionomeric resin made by E.I. DuPont de Nemours & Co., was introduced to the golf industry. Ionomers typically cost less than balata and have better cut or shear resistance. At the present time, ionomers are used as the primary polymer source for either or both of the cover stock and intermediate layers for most two-piece and some three-piece golf balls. The problem with ionomer-covered golf balls, however, is that they often lack the “click” and “feel” which golfers had become accustomed to with balata. “Click” is the sound made when the ball is hit by a golf club while “feel” is the overall sensation imparted to the golfer when the ball is hit.
However, unlike ionomer-covered golf balls, polyurethane- or polyurea-covered golf balls can be made to have the “click” and “feel” of balata and the cut or shear resistance of ionomer. Polyurethanes or polyureas are typically prepared by the reaction of a diisocyanate with a polyol (in the case of polyurethanes) or with a polyamine (in the case of a polyurea). Thermoplastic polyurethanes or polyureas may consist solely of this initial mixture or may be further combined with a chain extender to vary properties such as hardness of the thermoplastic. Thermoset polyurethanes or polyureas typically are formed by the reaction of a diisocyanate and a polyol or polyamine respectively, and an additional crosslinking agent to crosslink or cure the material to result in a thermoset.
One measure of a golf ball's performance is its resilience which is related to the balls Coefficient of Restitution (“COR”). The C.O.R. of a one-piece golf ball is a function of its composition. In two-piece golf balls and multi-layered golf balls, the C.O.R. is a function of the various properties of the core, the cover, and any additional layer. Although the United States Golf Association (U.S.G.A.) has not promulgated any limitations on the C.O.R. values for golf balls, it has instituted a rule prohibiting the competitive use in any U.S.G.A.-sanctioned event of a golf ball that can achieve an initial velocity greater than 76.2 meters per second (m/s), or 250 ft/s, when struck by a golf club driver having a velocity of 39.6 m/s, i.e., 130 ft/s (referred to hereinafter as “the U.S.G.A. test”). However, an allowed tolerance of two percent permits manufacturers to produce golf balls that achieve an initial velocity of 77.7 m/s (255 ft/s).
Players generally seek a ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, golf ball manufacturers strive to produce balls delivering initial velocities in the U.S.G.A. test that approximate the U.S.G.A. maximum of 77.7 m/s, or 255 ft/s, as closely as possible. Golf ball manufacturers also generally strive to maximize the ball's C.O.R. without violating the velocity limitation. Also, to maximize distance, it is advantageous if the balls have a lower driver spin rate. Finally it is highly desirable if, while providing increased velocity and distance, the balls also will exhibit a soft shot feel.
Recent multi-layer ball constructions have attempted to overcome some of the undesirable aspects of conventional two-piece balls, e.g., their hard feel. Such a multi-layer structure allows the introduction of new materials of varying hardness, whereby deficiencies in a property in one layer can be mitigated by the introduction of a different material in another layer. For example, to optimize ball hardness and “feel,” blends of copolymeric high-acid ionomers with softer terpolymeric ionomers have been used as a layer material in a golf ball but again, often with a concurrent loss of C.O.R. and/or speed.
Numerous examples of multi-layer combinations are available. For example, U.S. Pat. No. 4,431,193 discloses a golf ball having a multi-layer cover, in which the inner cover layer is a relatively hard, high flexural modulus ionomer resin and the outer cover layer is a relatively soft, low flexural modulus ionomer resin.
Also, U.S. Pat. No. 6,368,237 discloses a multi-layer golf ball comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a high-acid ionomer or ionomer blend. The outer cover layer comprises a soft, very low-modulus ionomer or ionomer blend, or a non-ionomeric thermoplastic elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layer golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
U.S. Pat. Nos. 6,416,424, 6,416,424, and 6,419,594, likewise, disclose multi-layer golf balls comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a low-acid ionomer blend. The outer cover layer comprises a soft, very low modulus ionomer or ionomer blend, or a non-ionomeric thermoplastic elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layer golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
U.S. Pat. Nos. 6,503,156 and 6,506,130, likewise, disclose multi-layer golf balls comprising a core, an inner cover layer, and an outer cover layer. The inner cover layer comprises a low-acid ionomer blend. The outer cover layer comprises a soft, non-ionomeric thermoplastic or thermosetting elastomer such as polyurethane, polyester, or polyesteramide. The resulting multi-layered golf ball is said to provide an enhanced distance without sacrificing playability or durability when compared to known multi-layer golf balls.
Another approach to optimizing golf ball performance has been to incorporate selected additives into the polymer compositions used to make the various ball layers, in order to modify the polymer properties. Such additives include the metal salts of various fatty acids. For example, U.S. Pat. Nos. 5,312,857 and 5,306,760 disclose cover compositions for golf ball construction comprising mixtures of ionomer resins and 25-100 parts by weight of various fatty acid salts (i.e., metal stearates, metal oleates, metal palmitates, metal pelargonates, metal laurates, etc.). However, the patents fail to disclose any major effects on ball properties, and fail to disclose that the compositions are useful for parts of a golf ball other than the cover.
Recent attempts to extend the concept of the use of multi-layer covers to mitigate the harsh feel of the harder ionomer materials have also resulted in the development of modified ionomers for use in golf ball compositions. For instance, U.S. Pat. No. 6,100,321 and U.S. Patent Publication No. 2003/0158312 A1 disclose ionomer compositions that are modified with 25 to 100 parts by weight of a fatty acid salt such as a metal stearate, for producing golf balls having good resilience and high softness. Unlike the earlier-mentioned patents, which have employed metal stearates as a filler material, U.S. Pat. No. 6,100,321 and U.S. Patent Publication No. 2003/0158312 A1 contemplates the use of relatively low levels of a stearic acid moiety, particularly metal stearates, to modify ionomers to produce improved resilience for a given level of hardness or PGA Compression values. The stearate-modified ionomers are taught as being especially useful when the ionomer is formulated for use as a golf ball core or center, as a one-piece golf ball, or as a soft golf ball cover. However, there is no disclosure of any ball construction parameters required to produce specific performance properties such as driver velocity or driver spin for three-piece balls.
Subsequent patents have furthered the use of such modified ionomers in golf ball covers. For example, U.S. Pat. No. 6,329,458 discloses a golf ball cover comprising an ionomer resin and a metal “soap,” e.g., calcium stearate. Finally, U.S. Pat. No. 6,616,552 discloses a golf ball including a multi-layer cover, one layer of which includes a heated mixture of an ionomer resin and a metal salt of a fatty acid, e.g., calcium stearate.
It should be appreciated from the foregoing description that there remains a need for a golf ball that can provide maximum C.O.R. without violating the velocity limitation. Also, to maximize distance, it is desirable for such balls to have a lower driver spin rate and to exhibit a soft shot feel. The present invention satisfies this need.